Heretofore, plastics have been widely used in the form of thin sheet-like or sheet-form sliding members such as tape pads employed as components of magnetic tape cassettes and slip sheets for floppy disks.
For instance, the tape pads are thin sheet-like components designed to prevent sagging and "coiling-around" of magnetic tapes running through cassettes. Such tape pads have hitherto been formed primarily of PET films.
For instance, proposals have been made of antistatically treated PET films or materials comprising backing PET films and about 100 to 200 .mu.m thick thin films of ultra-high-molecular-weight polyethylene or polytetrafluoroethylene (PTFE for short) which are antistatically treated in consideration of abrasion resistance and bonded to their regions of contact with magnetic tapes (see Japanese Patent Application Kokai Nos. 57(1982)-37950, 56(1981)-105373 and 56(1981)-10537). Generally, since plastics are likely to be electrified, means are often taken for escaping charges generated by their contact with tape surfaces by adding to them antistatic additives such as carbon.
Usually, the tape pads should have such properties as set out below:
(1) Suitable tension to prevent sagging of magnetic tapes during their transportation;
(2) Abrasion resistance to prevent the pads and the associated tapes from wearing off;
(3) Frictional force sufficiently low to ensure smooth traveling of tapes; and
(4) Sliding surfaces with a curvature sufficient to prevent tapes from being bruised.
Heretofore, films or sheets of ultra-high-molecular-weight polyethylene have generally been made by the following techniques:
(a) Films or sheet-like pieces are cut out of sheet or rod-like molded parts obtained by compression molding;
(b) Ultra-high-molecular-weight polyethylene is dissolved in an organic solvent, followed by film or sheet forming by casting;
(c) Ultra-high-molecular-weight polyethylene, to which an organic solvent has been added, is extruded, and the extruded product is then formed into a film or sheet by the volatilization of the organic solvent (Japanese Patent Publication No. 63(1988)-19327).
Problems with the technique (a) are that it is very poor in productivity and provides sheets having unsatisfactory surface smoothness. Drawbacks with the technique (b) are that a solution of ultra-high-molecular-weight polyethylene is not only so high in viscosity that it is difficult to handle, but is also so unstable that crystals, for instance, can precipitate out depending upon the temperature conditions selected. This is also a low commercial productivity and so it is not said to be a satisfactory method. Comprising three steps of adding the organic solvent to ultra-high-molecular-weight polyethylene, extruding that ultra-high-molecular-weight polyethylene and removing the solvent from the resulting sheet, the technique (c) is much more simplified in the steps involved than the above techniques (a) and (b) and offers significant advantages over them. Problems with this technique, however, are that the extrusion of the solvent-containing resin is so accompanied by heating that there can be the fear of inflammation, the step of removing the solvent by volatilization is not energy-effective, and the recovery of the solvent is time-consuming and involves much labor.
Some problems with such conventional tape pads originate from the properties of the materials used and some arise from the processing techniques applied. Of note in connection with the materials used is that in view of the principles of friction or tribology, a material of larger momentum should basically be harder than its associated material of smaller momentum. In other words, a tape material (PET film) should fundamentally be harder than its associated tape pad. Under such conditions, the amount of abrasion is considered to be minimized. It is also found that when materials that contact each other are of the same type, their relative abrasion is most likely to be increased.
In view of tribology, however, the materials used for conventional tape pads, especially PET films which are used per se as tape pads, are not in ideal conditions at all. This is because marked wear takes place between the polymeric material for a tape pad and the base film of a magnetic tape, both formed of PETs.
Polytetrafluoroethylene (PTFE), on the other hand, has a low coefficient of friction and is ideal in this sense, but poses a wear resistance problem that it cannot stand up to long-term use, because of its own property of keeping its coefficient of friction low by self-wearing.
Typical means taken today to avoid such a problem are set forth in Japanese Patent Application Publication Nos. 1982-37950, 1981-105373 and 1981-105374, in which highly wear-resistant films such as ultra-high-molecular-weight polyethylene films are used for the surfaces of tape pads to contact tapes together with PET supports or backings, thereby preventing the tape pads from being worn away and the tapes' magnetic surfaces from being bruised.
Required for the achievement of this, however, is the additional step of bonding the ultra-high-molecular-weight polyethylene films to the PET films, which offers problems in connection with mass production and cost effectiveness.
Now, forming sheets of ultra-high-molecular-weight polyethylene is still carried out with sheet-forming techniques requiring post-processing, for lack of any effective technique for forming sheets by single-stage extrusion.
Heretofore, extruding polyethylene into sheets has generally been performed with a polyethylene feed having a molecular weight lying between 20,000 and 200,000. It is known in the art that polyethylene having such a molecular weight can be extruded with relative ease, but difficulty is encountered in extruding sheets from polyethylene having a molecular weight exceeding that upper limit.
Problems with the above polyethylene having a molecular weight between 20,000 and 200,000, however, are that it is inferior in wear resistance and impact resistance to ultra-high-molecular-weight polyethylene such as polyethylene having a molecular weight exceeding 200,000. As techniques capable forming such ultra-high-molecular-weight polyethylene into sheets, on the other hand, ram extrusion has been mainly used but this can only can yield thick-walled products having a thickness of, say, 1 mm or higher.